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Date: Mon, 08 Apr 2013 18:18:50 -0400 From: KOSAKI Motohiro <kosaki.motohiro@...il.com> To: Cody P Schafer <cody@...ux.vnet.ibm.com> CC: KOSAKI Motohiro <kosaki.motohiro@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 2/3] mm/page_alloc: convert zone_pcp_update() to use on_each_cpu() instead of stop_machine() (4/8/13 3:49 PM), Cody P Schafer wrote: > On 04/08/2013 12:26 PM, KOSAKI Motohiro wrote: >> (4/8/13 1:32 PM), Cody P Schafer wrote: >>> On 04/07/2013 08:39 AM, KOSAKI Motohiro wrote: >>>> (4/5/13 4:33 PM), Cody P Schafer wrote: >>>>> No off-cpu users of the percpu pagesets exist. >>>>> >>>>> zone_pcp_update()'s goal is to adjust the ->high and ->mark members of a >>>>> percpu pageset based on a zone's ->managed_pages. We don't need to drain >>>>> the entire percpu pageset just to modify these fields. Avoid calling >>>>> setup_pageset() (and the draining required to call it) and instead just >>>>> set the fields' values. >>>>> >>>>> This does change the behavior of zone_pcp_update() as the percpu >>>>> pagesets will not be drained when zone_pcp_update() is called (they will >>>>> end up being shrunk, not completely drained, later when a 0-order page >>>>> is freed in free_hot_cold_page()). >>>>> >>>>> Signed-off-by: Cody P Schafer <cody@...ux.vnet.ibm.com> >>>> >>>> NAK. >>>> >>>> 1) zone_pcp_update() is only used from memory hotplug and it require page drain. >>> >>> I'm looking at this code because I'm currently working on a patchset >>> which adds another interface which modifies zone sizes, so "only used >>> from memory hotplug" is a temporary thing (unless I discover that >>> zone_pcp_update() is not intended to do what I want it to do). >> >> maybe yes, maybe no. I don't know temporary or not. However the fact is, >> you must not break anywhere. You need to look all caller always. > > Right, which is why I want to understand memory hotplug's actual > requirements. > >>>> 2) stop_machin is used for avoiding race. just removing it is insane. >>> >>> What race? Is there a cross cpu access to ->high & ->batch that makes >>> using on_each_cpu() instead of stop_machine() inappropriate? It is >>> absolutely not just being removed. >> >> OK, I missed that. however your code is still wrong. >> However you can't call free_pcppages_bulk() from interrupt context and >> then you can't use on_each_cpu() anyway. > > Given drain_pages() implementation, I find that hard to believe (It uses > on_each_cpu_mask() and eventually calls free_pcppages_bulk()). > > Can you provide a reference backing up your statement? Grr. I missed again. OK you are right. go ahead. > If this turns out to be an issue, schedule_on_each_cpu() could be an > alternative. no way. schedule_on_each_cpu() is more problematic and it should be removed in the future. schedule_on_each_cpu() can only be used when caller task don't have any lock. otherwise it may make deadlock. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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